CFD-DEM Numerical Simulation Of Gas-solid Two-phase Flow And Heat Transfer Characteristics During Vapor Deposition Of Pyrolysis Carbon For Artificial Heart Valve

Heart is one of the most important organs to maintain normal life activities,congenital malformations and acquired heart valve problems due to rheumatic heart disease,heart degeneration and bacterial infections will lead to abnormal heart valve function.Currently,prosthetic heart valve replacement is the main method of treating heart valve disease.And,the most clinically used all carbon bilobed prosthetic heart valves are produced by FB-CVD at a preparation temperature of 1200-1500°C via coating the surface of an isotropic graphite substrate with LTIC with excellent properties.However,the fluidization state of the fluidized bed inside the furnace and the occurrence of chemical reactions during the FB-CVD pyrolytic carbon deposition process affect the quality of the pyrolytic carbon coating.So far,due to the intense thermal cracking and polymerization reactions in the furnace and the influence of electromagnetic fields and high-temperature radiation when monitoring the furnace conditions using experimental methods,resulting in a lack of clear understanding of the velocity and temperature fields,etc.in the FB-CVD process.Therefore,it is necessary to investigate the real gas-solid flow and heat and mass transfer phenomena in the fluidized bed during FB-CVD pyrolysis carbon deposition.To this end,in this paper,a fast and low-cost numerical simulation method is chosen,which uses a trajectory-averaged coarse-grained CFD-DEM（CGM）method coupled with a heat transfer model that is more consistent with the actual particle fluidization process.In the pyrolytic carbon chemical vapor deposition,Zr O₂hollow particles play the role of bed particles and heat carriers,so this paper takes Zr O₂hollow particles as the solid phase,and conducts numerical simulation studies on the dense gas-solid two-phase flow,mixing characteristics and heat transfer characteristics in the steady-state fluidized bed,and the research content mainly contains the following three parts:（1）The discrete element method Hertz-Mindlin（no slip）contact model was chosen and the simulation parameters are calibrated by comparing the stacking angles of Zr O₂hollow spheres measured by physical and simulation tests,and the friction and recovery coefficients between Zr O₂hollow particles and between Zr O₂hollow particles and graphite materials are obtained,which fills the gap of Zr O₂granular material parameters in the discrete element software.（2）The effect of fluidizing wind speed on the mixing characteristics（particle distribution,particle motion and Lacey mixing index）of Zr O₂hollow sphere particles was investigated from the particle perspective using the CGM method.The results show that increasing the gas flow rate within a reasonable range in the fluidized bed could accelerate the mixing process,and the radial particle mixing process is more sensitive to the fluidized air velocity than the axial one.In addition,due to the different densities of particles along the radial direction in the furnace,the number of particle collisions and the influence of the traction force are also very different,and the particle velocity variation in the fountain area is more obvious,while the velocity distribution in the annular gap area near the wall is smoother.（3）The CGM method with coupled heat transfer model was used to study the effects of fluidization air velocity on the gas-solid heat transfer characteristics.It was found that the inlet gas velocity had a significant effect on the temperature distribution and heat transfer of the particles within the bed.The inlet gas velocity has a linear effect on the average particle temperature,and the increase of the inlet gas velocity directly promotes the heat transfer process within the bed.Moreover,the PDF distribution of particle temperature was narrower at 6.0 m/s than at 8.0 m/s under the study conditions in this paper.In addition,it was found that the particle-fluid convective heat transfer accounted for about 50%of the total heat transfer and contributed the most in the FB-CVD pyrolysis carbon deposition process,and the heat flux generated by the conduction heat transfer mode accounted for a smaller share of the total heat transfer.